The present invention relates to an electrophotographic developer and a production process thereof, and particularly to an electrophotographic developer which is excellent in flowability and shelf stability, does not give image defects such as occurrence of white stripes and attains almost constant image quality even when the ambient conditions including temperature and humidity fluctuate, and a production process thereof.
The present invention also relates a production process of a polymerized toner contained in the electrophotographic developer which exhibits such excellent properties or characteristics. The present invention further relates to an image forming process of the electrophotographic system making use of the electrophotographic developer.
In an image forming apparatus such as a copying machine or printer of the electrophotographic system, an electrostatic latent image formed on a photosensitive member is developed with a developer. The developer image formed on the photosensitive member is transferred to a transfer medium such as paper or OHP sheet as needed, and then fixed to the transfer medium by a method such as heating, pressing or use of solvent vapor.
As the developer, is used colored particles (toner) comprising a binder resin and a colorant. Developers are roughly divided into one-component developers with the surface of a toner coated with an external additive (flowability-imparting agent) such as silica and two-component developers composed of a toner and a carrier. The developers include magnetic developers making use of a magnetic toner comprising magnetic powder and non-magnetic developers making use of a toner comprising no magnetic powder. Non-magnetic one-component developers are recommended from the viewpoints of the miniaturization and weight saving of the image forming apparatus, and definition of images, etc.
There is yearly a strong demand for the improvement in the definition of images formed by the image forming apparatus of the electrophotographic system. As toners, toners (pulverized toners) obtained by a process (grinding process) comprising melting and mixing components such as a synthetic resin and a colorant and then grinding and classifying the resultant mixture to prepare colored particles have heretofore been used mainly. In recent years, attention has been attracted to toners (polymerized toners) composed of colored polymer particles obtained by a process in which a monomer composition comprising a polymerizable monomer and a colorant is subjected to suspension polymerization because they are easy to control their particle diameter, spherical and excellent in flowability and permit omitting a classification step.
Electrophotographic developers are required to have such characteristics or properties that they are excellent in flowability and shelf stability, do not give image defects such as occurrence of white stripes and attain almost constant image quality even when the ambient conditions including temperature and humidity fluctuate. However, it has been difficult to produce a toner and a developer satisfying these characteristics or properties, and various improved proposals have been made under the circumstances.
Japanese Patent Application Laid-Open No. 8-248676 has proposed an electrophotographic polymerized toner which is obtained by polymerizing a polymerizable monomer and has the relationship of 5 xcexcS/cmxe2x89xa6D2xe2x88x92D1xe2x89xa650 xcexcS/cm, wherein D1 is an electrical conductivity of water, and D2 is an electrical conductivity of a filtrate obtained by dispersing the polymerized toner in the water in a proportion of 1 g per 20 ml of the water, fully stirring the dispersion to equilibrium and then filtering off the polymerized toner.
The publication shows that when this polymerized toner is mixed with a carrier composed of ferrite particles coated with a resin to use it as a two-component developer, a change in charge level is little even under different environments of low temperature and humidity and high temperature and humidity. In other words, this polymerized toner is charged by friction with magnetic iron powder.
When this polymerized toner is used as a non-magnetic one-component developer in an image forming apparatus of a system that the toner is charged by friction with a development roller or a development blade, however, the dependence of charge level on environment becomes high, and deterioration of image quality by environmental changes is observed. This toner is also insufficient in flowability and shelf stability.
Japanese Patent Application Laid-Open No. 11-72949 has proposed an electrophotographic developer comprising polymer particles (polymerized toner) for developer obtained by polymerizing a polymerizable monomer and an external additive attached to the surfaces of the particles, wherein the pH of a water extract obtained by a method (boiling extraction method) of dispersing the developer in ion-exchanged water having a pH of about 7 and boiling the dispersion is about 4 to 7.
This developer is greatly improved in deterioration of image quality by environmental changes and also improved in shelf stability and flowability. However, a further improvement is required to fully satisfy the requirement level for improvement in image quality.
It is an object of the present invention to provide to an electrophotographic developer which is excellent in flowability and shelf stability, does not give image defects such as occurrence of white stripes, can form high-quality images and attains almost constant image quality even by environmental changes of temperature, humidity and the like, and a production process thereof.
Another object of the present invention is to provide a production process of a polymerized toner suitable for use in such a developer.
A further object of the present invention is to provide an image forming process making use of the developer which exhibits such excellent properties or characteristics.
The present inventors have carried out an extensive investigation with a view toward achieving the above objects. As a result, the present inventors have conceived a developer comprising boron and/or phosphorus in a specified proportion. A polymerized toner comprising boron and/or phosphorus in a specified proportion is suitable for use as such a developer.
Such a polymerized toner can be produced by a process for producing a polymerized toner, comprising the step of subjecting a monomer composition comprising a polymerizable monomer and a colorant to suspension polymerization in an aqueous dispersion medium containing a dispersion stabilizer to form colored polymer particles, wherein at least one water-soluble compound selected from the group consisting of water-soluble boron compounds and water-soluble phosphorus compounds is caused to exist in the aqueous dispersion medium to conduct the suspension polymerization. The content of boron and/or phosphorus can be controlled within a preferred range by suitably performing post-treatment steps after the polymerization.
The polymerized toner obtained by such a production process can achieve a far excellent result when it is used as a one-component developer with an external additive added thereto, in particular, a non-magnetic one-component developer.
The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided an electrophotographic developer comprising at least one element selected from the group consisting of boron and phosphorus in a content of 0.1 to 100 ppm.
According to the present invention, there is also provided a process for producing a polymerized toner, comprising the step of subjecting a monomer composition comprising a polymerizable monomer and a colorant to suspension polymerization in an aqueous dispersion medium containing a dispersion stabilizer to form colored polymer particles, wherein at least one water-soluble compound selected from the group consisting of water-soluble boron compounds and water-soluble phosphorus compounds is caused to exist in the aqueous dispersion medium to conduct the suspension polymerization.
According to the present invention, there is further provided a process for producing an electrophotographic developer, which comprises recovering a polymerized toner composed of colored polymer particles in accordance with the production process described above and then adding an external additive to attach it to the surfaces of the colored polymer particles.
According to the present invention, there is still further provided an image forming process comprising the steps of developing an electrostatic latent image on a photosensitive member with an electrophotographic developer to form a developer image, transferring the developer image to a transfer medium and fixing the developer image on the transfer medium, wherein a developer comprising at least one element selected from the group consisting of boron and phosphorus in a content of 0.1 to 100 ppm is used as the electrophotographic developer.